1. Field of the Invention
The present invention relates to a molding method and apparatus for molding articles of expanded plastic material.
Molding methods for articles of expanded thermoplastic material have been known heretofore and especially methods for manufacturing articles obtained by agglomerating pre-expanded polystyrene beads.
Presently these methods are commonly used for manufacturing blocks, supports or shells for packaging food products, machinery or household electrical appliances etc. . . These products have excellent impact resistance, they are weather resistant and their cost is relatively low.
2. Description of the Prior Art
Usually, the known molding techniques include the following steps:
filling a molding cavity of a mold with pre-expanded polystyrene beads
injecting steam for heating said beads to the softening temperature of the polymer and above the boiling point of the porogeneous fluid which is contained in said beads. Said steam causes the beads to expand and bring them to a partially molded state causing them to agglomerate into a homogeneous cellular mass.
cooling the product obtained in a cellular state contained in the mold in order to prevent a second expansion or a distorsion of the molded article; and, once the mold is cooled
withdrawing the molded article from the mold, after which the mold can be heated again for the next molding cycle;
This method involves the use of a substantial heating system and means for rapidly cooling the mold in order that the two alternate steps, i.e. heating the mold and cooling thereof, do not increase excessively the manufacturing costs of the articles.
However, since the article is made from an excellent heat insulating material, this method results in a very limited production rate, and reduces drastically the efficiency and the profitability of the molding equipment.
Another disadvantage which affects this method is the consumption of energy. For example, when heating 0.150 kg of thermoplastic material, for producing a medium size article contained in the mold it is repeatedly necessary to heat, at each molding cycle, the 150 kg of the mold material itself. It can be realized that this mass ratio of about 1 to 1000, results in a considerable loss of energy which heavily affects the cost of the finished product.
In order to reduce the manufacturing cost it has been proposed to provide molds with multiple molding cavities. This approach is valid for small size articles only and on the other hand the design and the manufacturing of the molds which have to be machined accurately, from a good heat conducting material, such as aluminium, are very expensive.
Also the expenses for unavoidable servicing and maintenance of so complex equipment have to be taken into consideration. In order to carry out successive cooling operations, the mold is provided with internal passages which allow cooling water to circulate. If this water is not treated, calcar deposits thereof will block the injection nozzles and attack the walls of the mold or of its auxiliary equipment. Further, for economical reasons, the factories that are using the known method, use cooling systems which operate in close circuit and consequently include storage containers, tanks for water treatment, water cooling towers and pumping stations which considerably affect the production cost of the molded articles of expanded polystyrene produced in mass production.
In order to reduce the energy consumption due to the requirements, in the conventional molding methods, of alternatively heating and cooling a single mold, it has also been proposed to carry on the molding successively in two working stations. For instance, the method disclosed in the U.S. Pat. No. 3,676,033 (Buonaiuto) is achieved in a machine comprising a pre-molding station with a mold permanently heated and a finishing station with a mold permanently cooled. The hot mold is comprised by three main parts: a vertical frame which defines the vertical edges of the molded plate, a lower body which forms the bottom of the plate, and an upper body which forms the top of the plate. The cold mold is also comprised by three parts: a vertical frame identical to the corresponding frame of the hot mold, a lower body and an upper body which may slightly differ from the corresponding bodies of the hot mold in that they may comprise convex or concave molding cavities which are intended to be reproduced on the top or a bottom face of the molded plate. The machine further comprises a transfer mechanism for transferring the pre-molded plate from the pre-molding station to the finishing station.
In said method, the pre-molded article remains held within its lateral frame during the transfer operation and it is moved between two parallel platens which contact permanently its upper and lower faces, respectively, so as to prevent the pre-molded article from further expanding outside the hot mold.
With such a machine it is only possible to manufacture molded pieces which originally have the form of plane plates, even if said plane articles may be ulteriorly shaped to different configurations.
Further, with such a molding method, mechanical means (rigid frame and parallel platens) are provided for preventing the rough article pre-molded in the hot mold of the pre-molding station from further expanding during its transfer towards the cold mold at the finishing station.
Such an operating mold presents many drawbacks:
the fabrication is limited to simple geometrical shapes. It is thus impossible to manufacture cylindrical or frustro-conical articles such as thin walled tumblers or cups, ice-boxes, flower-stands or the like, since the shape which can be obtained has to be developable from a plane plate;
expansion of the pre-formed articles is interrupted, said articles being constrained within a rigid frame during their transfer, whereby resulting in the production of objects which show relative poor elasticity and which are rigid and breakable;
the transfer apparatus is quite sophisticated, since the parallel platens, within which are slidingly received the frames holding the preformed plates, has to be carefully machined and fitted.